CT imaging is based on an irradiation of the object under investigation through a sample plane from different projection directions with X-rays created with a source device, followed by the reconstruction of the sample plane on the basis of attenuation data measured with a detector device at the different projection directions. Reconstruction of a complete image is based on collecting projection images with projection angles covering at least 180°. The object is arranged on a carrier device. For setting the different projection directions, the combination of the source and detector devices and the carrier device are capable of a scanning movement relative to each other. Various scanning techniques for implementing the scanning movement have been developed in the past.
With the first generation CT scanner, the source device comprises one single pencil beam source and the detector device comprises one single X-ray detector. A so-called rotation-translation-system is obtained with the following procedure. Starting at a particular angle, the source-detector-system is translated linearly across the field of view (FOV), wherein the data over parallel rays across the FOV are acquired for the projection at that particular angle. After completing the translation, the whole system is rotated, and then another translation is used to acquire data of the next projection direction. These steps of translation and rotation are repeated until the complete set of projection directions has been acquired. In the second generation CT scanner, the detector device comprises a linear array of a few detectors, whereas the X-ray tube creates a narrow fan angle X-ray beam. As in the first generation scanner, the scanner of the second generation is a rotation-translation system which however has a reduced number of rotation steps.
A major limitation of the first and the second generation CT scanners is the translation motion because at the end of each translation, the source-detector-system has to be stopped, the whole system has to be rotated and then the translation motion has to be restarted. The construction of fast scanning devices proved very difficult with these CT scanners. Therefore, first and second CT scanners are not used for current CT imaging.
The third CT scanner, as disclosed e.g. in WO 2007/034357 or U.S. Pat. No. 4,149,079, is characterized by a rotation-rotation-system, referring to the rotation of the source and detector devices. A generic third generation CT scanner 100 is schematically illustrated in FIG. 8 (prior art). The conventional CT scanner 100′ comprises a measurement device including a source device 20′ and a detector device 30′ as well as a carrier device 4′ accommodating the object 1′ under investigation. The source device 20′ comprises a single X-ray tube irradiating the object 1′ with an X-ray fan beam. The detector device 30′ comprises a plurality of detector elements detecting the radiation transmitted through the object 1′. The source and detector devices 20′, 30′ have a fixed position relative to each other. For setting the various projection directions, the source and detector devices 20′, 30′ are rotated around the object 1′, i.e. the spatial orientation of the measurement device including the source device 20′ and the detector device 30′ is continuously changed during scanning.
As an essential advantage, the third generation CT scanners are capable of providing essentially shorter scan times. A complete set of attenuation data required for CT image reconstruction can be collected within some milliseconds. However, conveying of signals from the detector elements requires wires from the detector device 30′ to a processing computer or the provision of contact rings for data and power transmission. Wiring has the disadvantage that problems may arise from the continuously changing spatial orientation of the measurement device. In particular, a continuous rotation is impossible, while the contact rings may cause mistakes in calibration of the detector signals. As a result, so-called ring artifacts can be created in the reconstruction image. As a further disadvantage, the efficiency of the fan beam geometry of data collected in the scanner of the third generation is computationally lower than that of the parallel beam geometry.
With the fourth generation CT scanner, the detector elements are removed from the rotating system and are placed on a stationary annulus around the object. In this case, the wiring and ring artifact problems of the third generation CT scanner can be avoided. However, the CT scanners of the fourth generation have an essential disadvantage in terms of their high price. This is due to the large number of detector elements required to form a complete ring. Another drawback of the fourth CT scanners may result from a non-homogeneity of the X-ray geometry. The source-detector distance as well as the thickness of the rays may be different for different detector elements. This may result in further imaging artifacts.
Further CT scanner have been proposed, which require mechanic structures being even more complex compared with the third or fourth generation CT scanners. As an example, SU 766 264 A1 discloses a scanning mechanism having a measuring device with one single X ray source and for each object to be measured one single detector element. For collecting data for multiple projection directions, the object is translated and rotated between the source and the detector element, i.e. the spatial orientation of an object carrier is continuously changed during scanning. Further scanners having a rotating object carrier are described in U.S. Pat. No. 5,119,408 A and WO 02/056752 A2.
Contrary to CT imaging, conventional tomosynthesis imaging can be adapted for collecting projections images during a straight translation of the object between the source and the detector element (e.g. US 2007/0116175 A1). However, this covers a limited range of projection angles only, so that a reconstruction of a complete tomographic image is impossible with the tomosynthesis imaging technique.